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(Callitriche) is a pretty and common plant, which may be use- 

 fully added to the aquarium. It is very variable, sometimes 

 creeping along in the mud, and at other times living entirely 

 submerged. The leaves are opposite, the lower ones much 

 narrower than those nearer the surface, while the terminal ones 

 form a star at the summit of the stem. Several species have 

 been recognised, but how far these are constant is not known. 

 The simple green flowers occur in the axils of the leaves. No 

 special arrangement is made for the winter, though the plant 

 may sink down to the level of the mud. Related is the horn- 

 wort (Ceratophyllum), which is, however, less common. It 

 floats freely in the water, and is thus like Anacharis a very 

 useful inhabitant of a temporary aquarium, devoid of sand or 

 mud at the bottom. The leaves are much divided, and com- 

 pletely submerged. They are arranged in whorls, and can be 



FlG. 42. Lesser bladderwort (Utricularia minor). The little swellings are the 

 bladders or traps in which small aquatic animals are captured. 



distinguished from those of Myriophyllum (see below) by the 

 fact that they are not pinnately divided, but once or twice forked. 

 The plant owes its name to the horny appearance of the old leaves. 

 A very interesting plant, belonging to the same family as 

 the little Pinguicula, or butter wort of bogs, is the bladderwort 

 (Utricularia). It is nowhere very common, but on the other hand 

 it is widely distributed, and may be looked for in deep pools. 

 Bentham and Hooker recognise three British species, differing 

 chiefly in size. Bladderwort floats freely in the water, and thus 

 forms an admirable aquarium plant. It has no true roots, 

 and consists of long (from a few inches to a foot or more) 

 floating root-like stems, furnished with numerous filamentous, 

 much-divided leaves, which bear the characteristic bladders. 

 In these bladders the plant captures small aquatic crustaceans 

 they are so arranged that entrance is easy but exit impossible, 



